Vacuum Forming Splitter

[Chip42]

plywood, attached from the top up to something like I showed will allow the splitter plank to break/break off without taking the bumper and other bits with it.

treat the splitter plate as disposable / expendable and you are halfway there.

[Eagle7]

Conceptually, that's exactly what I've got now, except I've got the front slot facing forward - which of course eliminates the possibility of a clean break. It took a big hit last summer and didn't fare well.

Do you have ideas for a rear pin that will shear when you want but not when you don't? Maybe something non-metalic and brittle? Or plastic?

[Chip42]

Whatever you choose it has to be strong enough to support the aero load and light dings but break free at a somewhat limitted contact force.

Pins dont really cover those criteria if you are making good aero use of the thing, ive been thinking about T6 aluminum flat stock, notched. But i have to run the math to see how much to notch and what cross section does best. Its not as easy to use as a bolt, but should give the desired results.

[Greg Amy]

What about shear bolts from a snowthrower? Made to shear under sharp force and you can buy them in six-packs at Lowe's. Just gotta design it so the shear plane is in the right location. - GA

[Chip42]

my thinking is that you need to have the aero-loading capability and the breakaway, so a pin, being equal strength in both rearward and downward loads (loaded in shear, mounted laterally) would need to be stronger than you might want for impact in order to be strong enough for use in general. but then there's the notion of orienting the bolt vertically...

[Greg Amy]

Understand, your vertical idea might work...but I think you're overstating just how much "downforce" these things provide, versus just reducing underbody lift. I'd be shocked if there's more than 50-75 pounds vertical force total on that plane...time for a fish scale, string, and GoPro camera?

[Chip42]

might be. the good news is that the exposed area in the front builds a lot of positive pressure on top, which could be creating moment about the forward pin, and actually negating a lot of the loading to the rear pin from whatever low pressure there is.

I think an average pressure drop underneath + front end high pressure area is producing ~150-200# on the front wheels of our little MR2s, just based on the bowing of the 3/4" hardwood plywood planks we use. we don't have good ride height change data. But there's a LOT of exposure under the beak of an MR2, so it might be a more extreme example (though most of you all are seeing 50% higher top speeds and should expect a LOT higher loading for a given shape).
EDIT:
throwing some math at it, and rough numbers for all variables, making assumptions etc... the splitter on our MR2, which has ~6in of distance from the tip to the first support bolt would take ~20psi of pressure differential to deflect the tip 0.1in.
Damn I wish I had me some CFD...